Manufacturers of electronic components use a variety of processing techniques to fabricate semiconductor devices. One technique that has many applications is known as "plasma-assisted" processing. In plasma-assisted processing, a substantially ionized gas, usually produced by radio-frequency (RF) or microwave electromagnetic gas discharge, generates a mixture of ions, electrons, and excited neutral species, which can react to deposit or etch various material layers on semiconductor substrates in a semiconductor processing reactor. Reactive ion etching (RIE), is an example of a widely used plasma-assisted semiconductor device fabrication process. RIE uses the directional energetic ions in the plasma to anisotropically etch materials on a semiconductor substrate. Various etching applications for plasma-assisted processing in semiconductor device manufacturing include isotropic and anisotropic etching of polysilicon, aluminum, and dielectric layers. Additional applications of plasma processing include plasma-enhanced chemical-vapor deposition (PECVD) of dielectrics, aluminum, copper, amorphous and polycrystalline silicon and other materials. Plasma-enhanced metal-organic chemical-vapor deposition processes (PEMOCVD) have been used for high-rate deposition of aluminum and copper films for semiconductor chip device interconnection applications. Plasma-assisted deposition techniques have also been used for planarized interlevel dielectric formation, including procedures such as bias sputtering; and low-temperature epitaxial silicon growth, including low-energy bias sputtering and PECVD of epitaxial layers.
In RIE, radio-frequency (RF:13.56 MHZ frequency, for example) electromagnetic waves interact with the gas molecules at low pressures (P &lt;mTorr) and produce a plasma medium via gas discharge between two parallel flat or concentric metallic electrodes. In conventional plasma processing techniques such as RIE, there is a trade-off between processing rate and the semiconductor device quality. First of all, increasing the RIE processing rate or throughput requires greater plasma density and/or ion flux (current density) at semiconductor substrate surface. According to the conventional methods, a direct method to increase ion current density is to increase the RF electromagnetic power that produces the plasma between two plasma electrodes. In RIE, the semiconductor wafer is usually clamped against one of the electrodes. Increasing the RF power transmitted to the plasma medium, however, raises the peak energies of plasma ions incident on the semiconductor substrate. Energetic ions (e.g. as high as 100's of eV) produced in plasma processing media can reduce device manufacturing yield due to irradiation damage and incorporation of metallic and other contaminants into the semiconductor device surface (contaminants sputter etched from plasma electrodes and reactor walls). When this type of plasma-induced surface damage occurs, post-plasma surface cleaning and/or annealing processes are necessary to reconstruct the surface and minimize the adverse effects to semiconductor device performance and manufacturing yield. The known RIE processes, based on fluorocarbon chemistries can also leave nonvolatile deposits such as fluoro-hydrocarbons on the semiconductor substrate surface. Ultimately, the manufacturer must remove these undesirable surface deposits and contaminants from the semiconductor device.
The combined effects of irradiation damage and incorporation of various types of surface contaminants produce semiconductor devices with less than optimal performance characteristics and limit fabrication process yield. Thus, with conventional RF plasma-assisted processing techniques, increasing RF power to increase ion current density with the intent to raise the processing throughput can have serious detrimental side effects. The interactions of the energetic charged species within a plasma processing module with the plasma electrode chamber walls can be a major source of metallic contamination in plasma-processed wafers. If a method existed, however, to increase the ion flux density on the semiconductor substrate without also substantially increasing ion energies, then a manufacturer may increase plasma-assisted processing rates with negligible surface damage or substrate contamination problems.
Therefore, a need exists for a method and apparatus to increase ion density and flux near a semiconductor substrate during plasma-assisted processing without at the same time significantly increasing ion energy levels.
Another limitation of conventional plasma-assisted processes derives from the fact that, during these processes, plasma disperses throughout the fabrication process chamber. In so doing, it interacts with the fabrication reactor chamber walls. These walls contain various metals (e.g. Fe, Al, Ni, etc.) that the ions can remove via sputtering (physical etch) and/or chemical reactions, transport to the semiconductor device surface, and embed into the semiconductor substrate. As a result, further semiconductor device degradation occurs. Consequently, there is a need for a method and apparatus to prevent plasma interaction with fabrication reactor process chamber walls during plasma-assisted processing.
To remedy the above problems in plasma-assisted processing, manufacturers often use techniques known as "magnetron-plasma-enhanced" (MPE) processing. MPE processing basically entails applying a predetermined magnetic field in the proximity of the semiconductor wafer during the plasma-assisted process. During an MPE process, a suitable magnetic field interacts with the plasma and causes the plasma to appear as a confined bright gaseous ball enveloping the semiconductor substrate and centered thereat. As a result, the plasma ion density is greatest near the semiconductor wafer, and the plasma that the semiconductor substrate sees does not significantly interact with the process chamber walls. This technique provides improved gas excitation and higher plasma density than with conventional plasma-assisted processes. MPE processing raises the device processing rate and reduces semiconductor device degradation due to energetic ions and process chamber wall contaminants by increasing the ion flux, reducing the average ion energies, and making the plasma concentrate near the semiconductor device. Thus, MPE processing can produce higher semiconductor device processing rates without a substantial increase in plasma ion energies.
Conventional MPE apparatus designs employ two straight permanent magnet bars positioned near the semiconductor device in the fabrication reactor. These permanent magnets produce a magnetic field over the semiconductor device surface which has both transverse and longitudinal magnetic flux components. The relative magnitudes of these magnetic field components vary significantly over the wafer surface. These global magnetic field non-uniformities cause plasma density and ion flux non-uniformities over surface of the semiconductor wafer. Consequently, a semiconductor device layer deposition or etching resulting from these conventional MPE processes will also suffer from undesirable non-uniformities.
The transverse and axial (or longitudinal) flux components from conventional magnetrons in a typical MPE cylindrical processing chamber can be thought of as producing both radial and tangential magnetic field non-uniformities. By rotating the conventional MPE magnetron in the horizontal plane, a conventional MPE magnetron can produce a cylindrically symmetrical magnetic field when averaged over time; however, the rotation cannot by itself eliminate the radial field non-uniformity problems. Rotating conventional MPE magnetrons in this manner requires a more complex fabrication reactor, because the magnetron platform is usually placed within the processing reactor and it must be mechanically driven by some rotating motor or other apparatus. This mechanical apparatus must be monitored and maintained during plasma-assisted processing to ensure that the conventional MPE processing module produces a cylindrically uniform transverse magnetic field distribution (when averaged over time).
Even with rotation, the conventional MPE reactors do not usually provide excellent radial uniformities for the magnetic flux density and the etching or deposition processes. Using the conventional MPE systems, the processing uniformity can only be improved or optimized by adjusting other process parameters such as the plasma RF power and/or process pressure. However, these process parameters, and their variations also affect the processing rate and other important characteristics of the MPE etch and deposition processes (e.g. line width control and etch selectivity in anisotropic etch applications). As a result, there is a need for a method and apparatus that can provide MPE processing with adjustable magnetic field strength and distribution over the semiconductor substrate surface.
While conventional MPE magnetrons may be able to produce a cylindrically symmetrical magnetic field because of the magnetic flux field shape they produce (and because of magnetic rotation), they cannot usually produce a radially uniform magnetic field. To overcome the limitations of a radially non-uniform magnetic flux field and/or nonuniform processing in conventional MPE magnetrons, one approach is to adjust the RF field strength from the RF power source. This adjustment, however, can adversely affect other process parameters associated with RF power. This is because, as already stated, varying the RF power affects the plasma ion energy levels. Variation of ion energy levels can affect selectivity and irradiation damage in etch processes and film stress and quality in deposition processes. As a result, there is a need for a magnetron plasma processing module which is capable of producing a suitable magnetic field distribution for both cylindrical and radial process uniformity in MPE processing of semiconductor devices.
Yet another limitation of conventional MPE processing magnetrons is their inability to make magnetic field distribution uniformity adjustments. The bar magnets of conventional MPE processing magnetrons produce a stationary magnetic field and do not permit flexible uniformity adjustment at different points on wafer surface without adversely affecting the magnetic flux density and distribution at other points near the semiconductor substrate. As a result, there is no capability to control the MPE process uniformity by varying and optimizing the magnetic flux field at various points near the semiconductor wafer. A need exists, therefore, for a method and apparatus that permits adjusting and optimizing the magnetic field distribution near the semiconductor wafer surface and thereby allows adjusting and optimizing MPE process uniformity.